The present invention relates to a voice signal transmitting system, and more particularly to a voice signal transmitting system which is of particular utility when employed in a communication system requiring high utilization efficiency of the transmission path, or a communication system which is subject to severe limitations on the transmission frequency band and transmitting power used.
Heretofore, 64 Kb/s PCM or 32 Kb/s APCM (Adaptive PCM) has been employed as a basic transmission path for digital transmission of voice signals. In this instance, coding of the voice signals at a low rate of 4.8 to 9.6 Kb/s without suffering appreciable deterioration of their quality would markedly improve the utilization efficiency of the basic transmission path and cut communication costs.
For economical construction of systems, which are severaly limited in terms of transmission frequency band, transmitting power and other transmission characteristics, such as a digital maritime satellite communication system, an air navigation satellite communication system, a digital business satellite communication system for business communications, and a digital mobile radio communication system for automobiles, there is a demand for a voice signal coding system which provides an excellent coded voice signal quality at a coding rate of about 4.8 to 9.6 Kb/s and is insusceptible to the influence of errors on the transmission path. The materialization of such a voice signal coding system will make it also possible to reduce a necessary storage capacity not only in the above-noted technical fields but also in a case where voice signals are stored in a coded form.
Hitherto, there has been proposed a residual Excited Linear Predicture coding system (hereinafter referred to as the "RELP system") as a typical coding system which employs the coding rate of 4.8 to 9.6 Kb/s.
The RELP system has its feature in that, by inputting an input voice signal into an inversed filter having a characteristic reverse from the correlation characteristic of the amplitude value of the input voice signal, a residual signal with a flattened short-time spectrum envelope is obtained and then the low-frequency component of the residual signal is transmitted after being coded into a waveform by PCM or adaptive delta modulation (ADM). On the receiving side, a high-frequency residual signal is regenerated by a non-linear reproducing method such as for rectification or a spectrum hold method of a spectrum folding principle, on the basis of the low-frequency residual signal obtained by waveform decoding. The low- and high-frequency residual signals are added together to restore the residual signal. The residual signal is applied as an exciting signal to a short-time spectrum synthesis filter, thereby reproducing a voice signal which has a spectrum envelope similar to that of the original voice signal.
In other words, the RELP system materializes the reduction of the coding rate by extracting the low-frequency component of the residual signal and transmitting it in the form of a waveform code.
By the way, for enhancement of the quality of the synthesized voice signal in the RELP system, it is important how faithfully the high-frequency components retaining the harmonic structure are reproduced on the synthesizing side. However, in the prior art which narrows the band of the low-frequency residual signal for decreasing the coding rate, as described above, the band of the high-frequency components to be reproduced broadens on the synthesizing side, and faithful reproduction of such high-frequency components is difficult, imposing a limitation on the enhancement of the voice signal quality.
As described above in detail, the defect of the conventional RELP system is atrributable to the basic arrangement which obtains a residual signal of a voice signal through inverse filtering, extracts therefrom a low-frequency residual signal, and transmits it after coding into a wave-form through the adaptive PCM (APCM) or adaptive delta modulation (ADM).